A power supply network comprises substations or electrical switchgears, in particular high or medium voltage switchgears which are controlled by a distributed substation control system. The substations comprise primary or field devices, e.g. switches, drives, generators, transformers, voltage and current transformers. The control system has, for example, field control devices and a control station which are connected to one another by means of various communication buses and bus couplers. The field control devices control, monitor and protect the field devices of the system.
Current transformers in electrical substations measure the system currents at predetermined measuring points of the switchgear with a certain measurement inaccuracy. The measuring points are typically located at all incoming and outgoing lines and possibly also within the system, e.g. for the busbar protection. The current measurement values are filtered, scaled to primary current values of the system, digitized, if necessary, and detected as current measurement signals by the control system. For communication purposes, additional rate-of-change filtering can be provided which e.g. comprises temporal averaging by integration or an algorithm for deciding about updating or retaining and transmitting or not transmitting the current measurement value. The current measurement signals are used for protective functions, for monitoring the substation, for calculating performance data for operating purposes or for consumption billing and for the representation on a display. Both the measurement values detected by the current transformer and the parameters of the filtering and scaling, i.e. the so-called parametrisation of the measurement values can have errors. Primary errors in the current transformer itself are produced e.g. by defective parts or material fatigue. Errors in the parametrisation or in the power calculation can be caused in the electronics by external influences, aging, drift, errors by the operating personnel or the like. Errors in the transmission of measurement values or measurement signals can occur in the device and function chain from the current transformer to the screen display or power system control center.
For protection purposes, it is known to monitor the current transformers by means of local plausibility tests at an isolated point of the facility. During the phase balance supervision test, all three phase currents and the neutral current are measured at one point in the line and deviations from an assumed maximum asymmetry between the phases are detected. During the comparison test of current and voltage values, coarse inconsistencies between voltage and current values can be discovered at the measuring point. These local plausibility tests related to a single measuring point are very coarse and only permit a yes/no decision, whether a current transformer is operating or not. Wrong scaling or a loss of accuracy, in contrast, cannot be detected, particularly since the tests in the protective device are performed, as a rule, with relative values to the nominal voltage.
In differential protection for transformers, the currents of the high-voltage end and the low-voltage end are measured, provided with a scaling factor given by the transformation ratio of the transformer and compared with one another. This makes it possible to detect and correct scaling errors for the differential protection.
Such tests are performed in protective devices for protective current transformers. In general, such algorithms cannot be used in control devices for measuring-type current transformers, because, as a rule, transducers are used for digitizing the current measurement values and these supply, instead of the instantaneous values needed, time-averaged RMS or effective values of the current, the voltage or the power, the frequency or the phase angle.